Stabilization of Pd, Pt and Ru nanoparticles by optically active CO/styrene copolymers

Article abstract of DOI:10.1016/j.inoche.2010.03.042

Fully isotactic, chiral CO/styrene copolymers were used as stabilizers for palladium, platinum and ruthenium nanoparticles synthesis. The tuning of copolymer/metal ratio and metal concentration were found to be key parameters in order to adjust the size and the agglomeration tendency of the metallic nanoclusters. These new materials were fully characterized by elemental analysis, infrared, solid state ¹³C MASNMRspectroscopies, and transmission electronic microscopy. Platinum and ruthenium nanoparticles were tested as catalysts in the ethyl pyruvate hydrogenation, giving high activities; in this preliminary work, no enantioselectivity induction was observed.

Full text of DOI:10.1016/j.inoche.2010.03.042

Inorganic Chemistry Communications 13 (2010) 766–768
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Stabilization of Pd, Pt and Ru nanoparticles by optically active CO/styrene copolymers
Isabelle Favier a, David Picurelli b, Christian Pradel a, Jérôme Durand c, Barbara Milani c, Montserrat Gómez a,
⁎
a
Laboratoire Hétérochimie Fondamentale et Appliquée, UMR CNRS 5069, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 9, France
Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès, 1-11, 08028 Barcelona, Spain
b
c
Dipartimento di Scienze Chimiche, Università di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Fully isotactic, chiral CO/styrene copolymers were used as stabilizers for palladium, platinum and ruthenium
nanoparticles synthesis. The tuning of copolymer/metal ratio and metal concentration were found to be key
parameters in order to adjust the size and the agglomeration tendency of the metallic nanoclusters. These new
materials were fully characterized by elemental analysis, infrared, solid state 13C MAS NMR spectroscopies, and
transmission electronic microscopy. Platinum and ruthenium nanoparticles were tested as catalysts in the
ethyl pyruvate hydrogenation, giving high activities; in this preliminary work, no enantioselectivity induction
was observed.
Received 1 February 2010
Accepted 27 March 2010
Available online 1 April 2010
Keywords:
Metallic nanoparticles
Copolymers
Isotacticity
© 2010 Elsevier B.V. All rights reserved.
Stabilization
Catalysis
Hydrogenation
The use of macromolecules (such as dendrimers [1–4], polymers [5–
10] or cyclodextrins [11]) in the preparation of metallic nanoparticles
for catalytic purposes represents a useful means to prevent the
agglomeration of these nanometric-scale catalysts towards the bulk
metal [12,13]. In order to apply these nanomaterials in asymmetric
processes without additional chiral modifier, the use of optically pure
polymers could be an attractive way to induce enantioselectivity. One
approach based on the stabilization of metallic nanoparticles by
polymer, introducing chiral groups in the polymeric chains was tested.
We have recently described Pd, Pt and Rh nanoparticles stabilized by
chiral ester or amide Gantrez® (registered trademark of GAF corpora-
tion for poly(methyl vinyl ether-co-maleic anhydride) derivatives [14].
These catalytic systems were active in Suzuki C–C coupling and
hydrogenation reactions. However, the chirality present on the lateral
chains of the polymer was inefficient to induce any asymmetry.
Another approach is based on the use of polymers featured by
main-chain chirality. With this aim, we planned to use isotactic,
optically active polymers as stabilizers of metallic nanoparticles. For
several years, some of us have extensively investigated the catalytic
synthesis of CO/vinyl arene copolymers [15–17], which belong to the
family of CO/alkene polyketones. Thanks to their physical, chemical
and mechanical properties CO/ethylene/propylene terpolymers were
exploited at industrial level under the trademark of Carilon® [18].
To the best of our knowledge no application of these copolymers
as stabilizers for metal nanoparticles has been reported so far. Here
we describe the first example of CO/styrene polyketones used as
stabilizers for Pd, Pt and Ru nanoparticles. In particular, fully isotactic,
optically active copolymers were employed to obtain active metal
nanoparticles as catalytic precursors in asymmetric hydrogenations
with the ambitious aim to transfer the chiral information from the
polymer to the prochiral substrate.
Platinum, palladium and ruthenium nanoparticles (MNP) were
prepared from the corresponding organometallic precursor, [M2(dba)3]
(for M=Pt, Pd; dba=dibenzylidenacetone) and [Ru(cod)(cot)] (cod=
1,5-cyclooctadiene; cot=cyclooctatriene), in the presence of isotactic,
optically active, CO/styrene copolymers under hydrogen atmosphere in
THF solution (Scheme 1), following the methodology previously
described [14,19,20]. Two CO/styrene copolymers, which were previ-
ously synthesized by us [21], were chosen differing for the number of
repetitive units in the chain (n=22 and 31, average values calculated
from the molecular weights bMwN determined by gel-permeation
chromatography versus polystyrene standards) and for the sign of the
optical activity (specific optical rotations measured in CHCl3 at 0.1 g of
polymer in 100 mL: +285 for 1 and −267 for 2); the stereochemistry of
these copolymers was determined by 13C NMR: only the signal related
to the ll triad was observed, indicating the formation of a fully isotactic
material [21].
The new materials were characterized by IR spectroscopy, elemental
analysis and transmission electronic microscopy (see Figs. S1 and S2
in Supplementary material). For all the metallic nanoparticles stabilized
by copolymers 1 and 2, their IR spectra (KBr pellets) showed the
characteristic absorption bands corresponding to the polymers, without
any shiftingrelative to thesignals of the pure polymer which proves that
no strong covalent interactions between polymer and metallic surface
were established. A new band at ca. 1000–1100 cm−1 was observed,
especially strong for Pt and Ru nanoparticles, absorption due to C–O
⁎
Corresponding author. Tel.: +33 561557738; fax: +33 561558204.
E-mail address: gomez@chimie.ups-tlse.fr (M. Gómez).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.03.042

I. Favier et al. / Inorganic Chemistry Communications 13 (2010) 766–768
767
Scheme 2. Ethyl pyruvate hydrogenation catalyzed by Ru2 and Pt2 systems.
and platinum could be explained on the basis of the different rate of the
nanoparticles nucleation and growth [23–26].
Scheme 1. Synthesis of Pd (Pd1), Pt (Pt1, Pt2) and Ru (Ru1, Ru2) nanoparticles.
As indicated above, polymer 2 was also used as nanoparticles
stabilizer. In particular, the effect of the monomer/metal ratio for
ruthenium was explored. Therefore, monomer/Ru ratios=0.5, 2 and
10, at a metal concentration of 2.5.10−2 M, were considered (TEM
images in Supplementary material). For the monomer/Ru=0.5, small
particles (mean diameter=1.22 nm) were obtained (Fig. S2a),
analogously to those prepared using the copolymer 1 as stabilizer
(Fig. S1f). For higher copolymer/metal ratios (monomer/Ru=2 and
10), the particles show a trend to be associated giving agglomerated
materials (Fig. S2b and S2c), in contrast to the effect observed when
PVP is used as stabilizer [27].
single bond stretching. The presence of this kind of bond was also
suggested by the solid state 13C MAS NMR spectra of Ru nanoparticles
(Fig. S3a and S3b). Actually, Ru1 and Ru2 showed large bands at ca. 80
and 45 ppm, in addition to those corresponding to the polymer,
assigned to C–O single bond and methylene groups respectively
(Fig. S3b and S3c vs S3a). In order to evidence the origin of these
bands, Ru nanoparticles stabilized by 1 were prepared in pentane
instead of tetrahydrofuran (Fig. S3d). The related 13C MAS NMR
spectrum did not exhibit the bands corresponding to C–O and additional
CH2 moieties proving that the signals at ca. 80 and 45 ppm are due to
tetrahydrofuran and not to the partial reduction of the carbonyl group of
the polymer in the presence of a metal source. The IR spectrum of Ru1
prepared in pentane also confirmed the absence of the C–O function.
Monomer/metal ratio and metal concentration effects were
studied in order to obtain small and well-dispersed nanoparticles
suitable for further catalytic applications in wet medium (Table 1)
[22]. An increase of the polymer proportion (monomer/metal
ratio=4.4) led to the formation of smaller nanoparticles depending
on the metal concentration employed.
In particular, for platinum materials, spherical associations of
nanoparticles of mean size of 22 nm were obtained at the most diluted
conditions used (entry 3, Table 1), while for ten times more con-
centrated conditions, these associations show a mean diameter ca.
40 nm (entries 1 and 4, Table 1); only agglomerates were observed at
10−2 M metal concentration (entry 5, Table 1). The same trend
was observed for the analogous Pd nanoparticles (entries 2, 6 and 7,
Table 1), but in this case, even for the most diluted conditions (entry 6,
Table 1), only large particle associations were formed. In order to
compare the stabilizing polymer effect, platinum nanoparticles were
also prepared with the copolymer 2, obtaining big nanoparticles
analogously to those containing 1 (entry 8 vs 1, Table 1). For ruthenium,
under the most appropriated conditions ([Ru]=2.5·10−2 M and
monomer/metal=2) very small and well-dispersed nanoparticles
were obtained for both 1 and 2 stabilizers (entries 9 and 10, Table 1).
This dissimilar behaviour between ruthenium in relation to palladium
Pt and Ru nanoparticles stabilized by copolymer 2 were used as
catalyst in the ethyl pyruvate hydrogenation (Scheme 2) (see
Supplementary material for experimental details).
Under the same catalytic conditions, Ru2 system was found to be
more active than that using Pt2 as catalyst (entry 1 vs 2, Table 2). For
higher metal load, the conversion of substrate increases up to 60%
(entry 3 vs 1, Table 2). However, no enantioselectivity induced by
the optically pure copolymer 2 was observed. The ethyl pyruvate
hydrogenation was also performed in the presence of cinchonidine
(entries 4 and 5, Table 2), a chiral inducer used in metal-catalyzed
pyruvate derivatives hydrogenation [28–31]. The addition of cincho-
nidine into the catalytic mixture resulted in an increase in the activity
of the system (for Ru, entry 3 vs 5; for Pt, entry 2 vs 5) [32,33], probably
due to the interaction between the substrate and the chiral modifier
favouring the contact of the substrate with the metallic surface [34–
36]. In addition, the basic nature of cinchonidine can also favour the
heterolytic cleavage of H2 induced by the metal. However, the product
was again obtained as a racemic mixture. Polyketone, close to the
metallic surface, probably avoids the approach of cinchonidine to the
metallic surface, losing the chiral induction.
To sum up, we have been interested in the metallic nanoparticles
stabilization by means of chiral copolymers with the aim of applying
them in asymmetric catalytic processes. New Pd, Pt and Ru nano-
particles could be prepared using fully isotactic, optically active CO/
styrene copolymers. The influence of parameters such as polymer
content and metal concentration has been considered in order to
control size and dispersion in the nanomaterials synthesis. Palladium
and platinum systems gave agglomerates or association of particles for
different monomer/metal ratios even at diluted metal concentration
(2.2.10−4 M). In contrast, ruthenium led to small spherical and well-
dispersed nanoparticles (mean diameter ca. 1.2 nm) for monomer/Ru
ratios up to 2 at relatively high metal concentration (2.2.10−2 M); at
Table 1
Mean size for Pd1, Pt1 and Ru1 nanoparticles in relation to the metal concentration and
monomer/metal ratio.
Entry
Precursor
[M](mol·L−1
)
Monomer/metal
Φm(nm)
1
2
3
4
5
6
[Pt2(dba)3]
[Pd2(dba)3]
[Pt2(dba)3]
[Pt2(dba)3]
[Pt2(dba)3]
[Pd2(dba)3]
[Pd2(dba)3]
[Pt2(dba)3]
[Ru(cod)(cot)]
[Ru(cod)(cot)]
2.2·10−3
2.2·10−3
2.2·10−4
2.2·10−3
2.2·10−2
2.2·10−4
2.2·10−3
2.2·10−3
2.5·10−2
2.5·10−2
0.5
0.5
4.4
4.4
4.4
4.4
4.4
0.5
2
38a
Aggl.b
22a
Table 2
Ethyl pyruvate hydrogenation catalyzed by Ru and Pt systems.a
39a
Entry
Catalystb
Added cinchonidine (%)c
Conv. (%)d
Aggl.b
37a
1
2
3
4
5
Ru2 (2%)
Pt2 (2%)
Ru2 (10%)
Ru2 (10%)
Pt2 (2%)
–
25
13
60
100
35
7
Aggl.b
35a
–
8c
9
–
1.13d (Ru55
1.22d (Ru70
)
)
10
10
10c
2
a
a
Spherical associations of nanoparticles.
Aggl. = agglomerates.
Nanoparticles stabilized by copolymer 2.
Reaction conditions: 0.5 mmol ethyl pyruvate, 10 mL ethanol, 40 bar hydrogen,
b
c
1 h, r.t.
b
c
In brackets, percentage of metal (in mol) in relation to the substrate.
Percentage of cinchonidine (in mol) with respect to the substrate.
Determined by 1H NMR.
d
In brackets, the estimated metallic composition considering a compact packing
d
arrangement of spherical nanoparticles.

768
I. Favier et al. / Inorganic Chemistry Communications 13 (2010) 766–768
[10] V. Thomas, M. Namdeo, Y. Murali Mohan, S.K. Bajpai, M. Bajpai, J. Macromol. Sci.
Pure Appl. Chem. 45 (2008) 107–119.
[11] A. Denicourt-Nowicki, A. Ponchel, E. Montflier, A. Roucoux, Dalton Trans. (2007)
5714–5719.
[12] A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757–3778.
[13] R. Shenhar, T.B. Norsten, V.M. Rotello, Adv. Mater. 17 (2005) 657–669.
[14] I. Favier, M. Gómez, G. Muller, D. Picurelli, A. Nowicki, A. Roucoux, J. Bou, J. Appl.
Polym. Sci. 105 (2007) 2772–2782.
[15] A. Scarel, M.R. Axet, F. Amoroso, F. Ragaini, C.J. Elsevier, A. Holuigue, C. Carfagna, L.
Mosca, B. Milani, Organometallics 27 (2008) 1486–1494.
[16] J. Durand, E. Zangrando, M. Stener, G. Fronzoni, C. Carfagna, B. Binotti, P.C.J. Kamer,
C. Müller, M. Caporali, P.W.N.M. van Leeuwen, D. Vogt, B. Milani, Chem. Eur. J. 12
(2006) 7639–7651.
[17] J. Durand, B. Milani, Coord. Chem. Rev. 250 (2006) 542–560.
[18] N. Alperwicz, Chem. Week (1995) 127.
higher copolymer contents aggregates were also formed. The solid
state characterization of Ru materials, by means of IR and NMR
spectroscopy, points to a weak interaction of the polymer with the
metallic surface, without polymer modification under the employed
synthesis conditions. Ru and Pt materials were tested as catalytic
precursors in ethyl pyruvate hydrogenation. Their catalytic activity
was considerably increased by addition of a chiral modifier (cincho-
nidine), mainly for ruthenium-catalyzed process, although no enan-
tioselectivity induction was achieved. Attempts to obtain chiral
polymers coming from copolymerization of cinchonidine with styrene,
carbon monoxide and acrylate derivatives were unsuccessful (see
Supplementary material).
[19] F. Dassenoy, M.-J. Casanove, P. Lecante, C. Pan, K. Philippot, C. Amiens, B. Chaudret,
Phys. Rev. B 63 (2001) 235407.
[20] Synthesis of metallic nanoparticles: The polymer was introduced in a Fischer-
Porter bottle and purged by means of vacuum and argon cycles. The appropriate
metallic precursor, [Pt2(dba)3], [Pd2(dba)3] or [Ru(cod)(cot)], was dissolved in a
Schlenk vessel with previously degassed THF and then introduced under argon
atmosphere in the Fischer-Porter bottle. The solvent volume was completed to
80 mL. The system was then pressurised with hydrogen (3 bars) and stirred at
room temperature overnight. The hydrogen was next replaced by argon and the
solvent was evaporated under vacuum. To eliminate the organic by-products, the
black residue corresponding to the formed nanoparticles was washed with
pentane and dried under vacuum.
Acknowledgments
The authors wish to thank Agence National de la Recherche
(Nanoconficat, project no. ANR-05-NANO-030), CNRS, Ministerio de
Educación y Ciencia (CTQ2007-61058/BQU) and European Network
“PALLADIUM” (5th Framework Program, HPRN-CT-2002-00196) for
financial support.
[21] A. Scarel, J. Durand, D. Franchi, E. Zangrando, G. Mestroni, C. Carfagna, L. Mosca, R.
Seraglia, G. Consiglio, B. Milani, Chem. Eur. J. 11 (2005) 6014–6023.
[22] J. Durand, E. Teuma, M. Gómez, Eur. J. Inorg. Chem. (2008) 3577–3586.
[23] J. Turkevitch, Gold Bull. 18 (1985) 125–131.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.03.042.
[24] M.A. Watzky, R.G. Finke, J. Am. Chem. Soc. 119 (1997) 10382–10400.
[25] M.A. Watzky, R.G. Finke, Chem. Mater. 9 (1997) 3083–3095.
[26] M.A. Watzky, E.E. Finney, R.G. Finke, J. Am. Chem. Soc. 130 (2008) 11959–11969.
[27] H. Hirai, N. Yakura, Y. Seta, S. Hodoshima, React. Funct. Polym. 37 (1998) 121–131.
[28] M. Studer, H.-U. Blaser, C. Exner, Adv. Synth. Catal. 345 (2003) 45–65.
[29] H. Bönnemann, G.A. Braun, Chem. Eur. J. 3 (1997) 1200–1202.
[30] X. Zuo, H. Liu, D. Guo, X. Yang, Tetrahedron 55 (1999) 7787–7804.
[31] J.U. Köhler, J.S. Bradley, Langmuir 14 (1998) 2730–2735.
[32] E. Toukoniity, D.Y. Murzin, J. Catal. 241 (2006) 96–102.
[33] M. Bartok, M. Sutyinszki, I. Bucsi, K. Felföldi, G. Szöllosi, F. Bartha, T. Bartok, J. Catal.
231 (2005) 33–40.
[34] J.L. Margitfalvi, E. Tálas, E. Tfirst, C.V. Kumar, A. Gergely, Appl. Catal. A General 191
(2000) 177–191.
[35] M. Schürch, T. Heinz, R. Aeschimann, T. Mallat, A. Pfaltz, A. Baiker, J. Catal. 173
(1998) 187–195.
[36] V. Mévellec, C. Mattioda, J. Schulz, J.-P. Rolland, A. Roucoux, J. Catal. 225 (2004)
1–6.
References
[1] M. Wilson, M.R. Knecht, J.C. García-Martínez, R.M. Crooks, J. Am. Chem. Soc. 128
(2006) 4510–4511.
[2] S.-K. Oh, Y. Niu, R.M. Crooks, Langmuir 21 (2005) 10209–10213.
[3] C. Ornelas, L. Salmon, J. Aranzaes Ruiz, D. Astruc, Chem. Commun. (2007)
4946–4948.
[4] T. Mizugaki, M. Murata, S. Fukubayashi, T. Mitsudome, K. Jitsukawa, K. Kaneda,
Chem. Commun. (2008) 241–243.
[5] D.E. Bergbreiter, J. Tian, C. Hongfa, Chem. Rev. 109 (2009) 530–582.
[6] A. Corma, H. Garcia, Top. Catal. 48 (2008) 8–31.
[7] J. Lu, P.H. Toy, Chem. Rev. 109 (2009) 815–838.
[8] D.E. Bergbreiter, S.D. Sung, Adv. Synth. Catal. 348 (2006) 1352–1366.
[9] R.B. Grubbs, J. Polym. Sci. Part A Polym. Chem. 43 (2005) 4323–4336.